Calcite twins from microveins as indicators of deformation history

In order to investigate whether the evidence of the deformation history is preserved in the microvein calcite twinning microstructures, several families of calcite veins have been studied from weakly deformed mesozoic limestones of the western Iberian Chain (eastern Iberian Peninsula). The maximum shortening directions established with this method correspond to the previously established paleostress evolution of this chain. The reconstructed stress regimes evolve from predominantly compressional (four compressional episodes, with the maximum shortening direction oriented successively NW–SE, N–S, NE–SW and E–W) to extensional. The established orientations of maximum shortening axes are consistent with the directions of paleostrains that have been independently deduced from stylolites and fault striations in the same region. Moreover, the results clearly show that the e-twins record some paleostress orientations that are not registered in the studied area by fault population analysis. In this sense, the e-twins record older paleostress orientations than the small faults, probably because the faults were reactivated several times and the twins were not modified since their formation. The calculated values of twin strains, ∼1%; differential stress magnitudes, 62 MPa; and the existence of type I twins formed below 200°C, confirm that this region was slightly deformed under low P–T conditions.